Open Cell Foam Metal Heat Exchanger

ABSTRACT

A method of enhancing an open celled foam metal heat exchanger is presented where the structure uses fluid channels that distribute fluid and/or air across a continuous flow field. The heat exchanger not only improves heat transfer properties given a required pressure drop but also takes into consideration the need to manufacture low cost solutions that may be mass produced to meet high capacity throughput requirements for the air and space industries.

TECHNICAL FIELD

Embodiments of the present invention relate to the technical field of heat exchangers. More particularly, the embodiments of the present invention are directed toward low cost—high performance heat exchangers that utilize open celled foam metal.

BACKGROUND OF THE INVENTION

Open celled foam metal materials have many uses. These materials have been engineered and manufactured as heat exchanger solutions. The primary use for open celled foam metal heat exchanger applications primarily resides within the air and space market segments due to the need for high performance and low weight requirements. Examples of where the product has been used as a heat exchanger includes satellite mirrors, computer heat sinks on aircraft, commercial space expeditions, powered electronics cooling, and the European Mars Rover, NASA.

Open celled foam metal materials have structures that generally take on the characteristics of a base alloy. These base alloys typically consist of low temperature alloys such as aluminum, copper, zinc, and other refractory metals. The advantages of an open celled foam structure is that the material offers high surface areas and outstanding strength to weight ratios. Unlike closed cell foams, gases, liquids, and other mediums may pass through the open pores of the material. This enables the material to be ideal for use as a phase change heat exchanger, air/liquid cooled heat exchanger, air to air heat exchanger, liquid to liquid heat exchanger, cold plates, and a number of other heat transfer applications that take advantage of the high surface area and the permeable lattice structure.

Open celled foam materials that are manufactured using a chemical vapor deposition (CVD) type processes utilize a host structure as a base for additive materials. These host structures are typically plastic or other materials that do not have the same composition as the CVD additive. The result is that CVD type manufactured foams are considered hollow and therefore result in lower thermal reduction properties due to significantly lower cross-sectional area. This material disadvantage reduces the performance of CVD type materials and presents limitations that are not encountered by open celled foam metals produced without CVD type processes.

Other additive manufacturing processes, including 3-D printing, have similar disadvantages. Most 3-D printing techniques create slip planes in between layers. Consequently, inconsistent temperature profiles during the additive manufacturing process create such degraded boundary layer effects. These factors cause lower thermal performance.

DUOCEL®, produced by ERG Aerospace since 1967, is an example of an open celled foam metal produced without CVD or other such additive manufacturing processes. As described in U.S. Pat. No. 3,616,814, production begins with a commercially available conventional foam, such as reticulated polyurethane having the desired pattern for the end product, that serves as a form. The conventional foam is embedded in mold material, such as plaster of paris, which sets to form a solid structure in and around the plastic foam. The structure is then heated to volatilize and expel the plastic foam, leaving voids in the mold corresponding to the original configuration of the foam. Molten metal is then cast through the voids in the mold and permitted to cool and set prior to washing away the mold structure. The resulting foam metal is a reticulated structure of integrally formed solid metal ligaments and open cells with pores connecting adjacent cells. The solid metal ligament structure provides improved properties compared to the hollow ligaments formed from additive manufacturing processes. Open celled foams can also be compressed further increasing the surface area to volume ratio. This type of compression is not possible with CVD or additive manufactured foam structures

Open celled foam metal materials, including DUOCEL®, are manufactured in a range of pore sizes. These sizes include 5 pores per inch (PPI), 10 PPI, 20 PPI, and 40 PPI. The advantage of having different pore sizes is that the material may be optimized for different applications based on the pressure drop requirements of a heat exchanger and the thermal performance. As an example, if high pressure drop is a primary requirement of an end user, then the 40 PPI material can be chosen to provide adequate pressure drop given the higher surface area. Conversely, a 5 PPI material may provide less pressure drop but have less thermal heat transfer leading to lower performance. Therefore, there is a pressure drop and heat transfer performance consideration for the specific open celled foam metal material chosen.

It is also possible to control the relative density of an open celled foam metal material, including DUOCEL®, for each of the pore sizes referred to above. In other words, it is possible to add material to the individual ligaments of the open celled foam metal to create relative density ranges. As an example, a 5 PPI piece of open celled foam metal may be modified at the individual ligament level to achieve relative density ranges anywhere from 3 to 20 percent relative density (relative to the weight of the solid alloy, or volume fraction of the metal). The relative density, much like the PPI, may be modified as a design parameter to meet end user requirements. This design customization further enables an open celled foam metal to meet precise customer pressure drop and thermal performance criteria.

While some open celled foam metal materials, such as DUOCEL®, have been available for a number of years, and allow customization to create high performance heat exchangers, costs associated with designing and manufacturing such improved heat exchangers has made the product less desirable compared to cheaper pin-fin style heat exchange systems.

BRIEF SUMMARY OF THE INVENTION

Therefore, there is a need to develop heat exchangers from suitable open celled foam metal materials, such as DUOCEL®, that are both high performance and affordable to manufacture. Doing so expands the use of such materials to include jet engines, cars, and electronic cooling structures where high demand may be achieved through optimum manufacturability. Doing so also enables such materials the ability to directly compete with 3-D printed structures, CVD type, and pin fin type heat exchangers currently available to the air and space market segments where high demand throughput may be achieved.

Furthermore, there is a need for improved heat transfer performance that is modular in design, given pressure drop for different energy level requirements. In other words, the heat exchanger design is scalable to meet a wide variety of different energy levels for both fluids and air. This includes a design for 1 kilowatt energy removal systems, 2 kilowatt energy removal systems, 3 kilowatt energy removal systems, and more.

It is a further objective of the present invention to create heat exchangers from open celled foam metal materials, such as DUOCEL®, that may be mass manufactured using vacuum brazing, dip brazing, or casting techniques. These standard techniques may be used for aluminum, copper, stainless steel, titanium, and other common and emerging heat transfer alloys.

It is yet a further objective of the present invention to provide methods of manufacturing flow channels that allow fluid distribution across a flow field of an open celled foam material, such as DUOCEL®. These flow channels ensure enhanced material coverage while reducing pressure drop but are designed to address pressure drop over length considerations.

It is a further objective of the present invention to use cross channel flow fields where each flow field includes the use of an open celled foam metal material, such as DUOCEL®, specific for a given pressure drop and heat transfer requirement.

It is a further objective of the present invention to consider the geometry of the individual ligament structures to improve flow across each individual ligament where turbulent and laminar flow fields differ given viscosity and Reynolds numbers.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention.

FIG. 1A is a perspective view of an open celled foam metal counter flow heat exchanger (1) for heat transfer given pressure drop. FIG. 1B is a perspective view showing a combination of open celled foam metal panels (4) where a combination of hot panels (2) and cold panels (3) are combined to create a counter flow heat exchanger.

FIG. 2A is a top view of the open celled foam metal counter flow heat exchanger (1) showing one embodiment of the direction of fuel and oil input and output. FIG. 2B is a top view of an individual hot panel (2) where the hot fluid inlet (5) and outlet (6) are shown.

FIG. 2C is a top view of an individual cold panel (3) where the cold fluid inlet (7) and outlet (8) are shown.

FIG. 3 is a perspective view of a single cell (15) of a relative density continuous one-piece insoluble reticulated open celled foam material prior to densification showing the ligament (16) and pore (17) structures.

FIG. 4 is a perspective view of a cell (15) from a relative density continuous one-piece insoluble reticulated open celled foam material after densification that has improved heat transfer given a certain pressure drop showing the ligament (16) and pore (17) structures.

FIG. 5 is a chart that shows the different geometries of individual ligaments that are considered for fluid flow given laminar and turbulent options; size bar is 1 mm.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The present invention will now be described in detail with reference to the accompanying drawings, wherein the same reference numerals will be used to identify the same or similar elements throughout the several views. It should be noted that the drawings should be viewed in the direction of orientation of the reference numerals.

In addition, while the embodiments illustrate liquid flow, heat exchange between hot and cool gases is also envisioned and encompassed by the invention. Therefore, the invention should not be viewed as limited to liquids.

FIG. 1A illustrates an open celled foam metal counter flow heat exchanger (1) for heat transfer given pressure drop. FIG. 1B illustrates a vertical cross section of the structure of the heat exchanger (1), which is comprised of a combination (4) of at least one hot panel (2) and at least one cool panel (3) enclosed in an impermeable container (10). The impermeable container (10) can be made of appropriate heat stable substances. In some embodiments the impermeable container (10) of the open celled foam metal counter flow heat exchanger (1) is made of heat stable substances that have insulating properties, including, but not limited to, A.B.S. (acrylonitrile, butadiene, and styrene), acetates, acrylics (e.g. ACRYLITE®, LUCITE®, plexiglass, etc.) ceramics (e.g. MACOR®, alumina, etc.), DELRIN®, epoxy/fiberglass, FEP, fiberglass laminates, high impact polystyrene (HIPS), KAPTON®, KAPTREX®, KYNAR®, melamine, MELDIN® 7001, mica, neoprene, NOMEX®, NORYL™, nylon, PEEK (polyether ether ketone), PET (polyethylene terephthalate), P.E.T.G., phenolics, PFA (perfluoroalkoxy), polycarbonate, polyester, polyolefins, polystyrene, polysulfone, polyurethane, TEFLON®, polyvinylchloride, REXOLITE® 1422 &2200, RYTON®, silicone/fiberglass, silicone rubber, TECHTRON®, ULTEM®, and VESPEL® SP-1. In some embodiments, materials capable of efficient heat transfer such as metals and metal alloys (e.g. aluminum, copper, brass, steel, bronze, etc.) are preferred. In yet other embodiments, however, metals such as stainless steel, alloys of iron and chromium, lead, and titanium are preferred.

In the embodiment shown, a cool liquid, such as fuel, enters a channel (12) along the outer edge of the cool panel (3) that is separated from the hot panel (2) by an impermeable barrier (11) and then passes through an open celled foam metal structure (9) before exiting into a channel (13) located on the other side of the open celled foam metal structure (9). Simultaneously, or in temporal proximity, a hot liquid, such as oil, enters a channel (12) along the outer edge of the hot panel (2) and passes through an open celled foam metal structure (9) in the opposite direction as the liquid flowing through the cool panel (2) before exiting into a channel (13) located on the other side of the open celled foam metal structure (9).

FIG. 2 A illustrates the liquid flow of one embodiment of the impermeable container of the open celled foam metal counter flow heat exchanger (1) as seen from the top. Here, the hot liquid (e.g. oil) enters the open celled foam metal counter flow heat exchanger (1) at the outside corner of one end of the heat exchanger (1) and exits at the opposite side and opposite end of the heat exchanger (1). Similarly, the cool liquid (e.g. fuel), enters the open celled foam metal counter flow heat exchanger (1) from the opposite corner of the same end as the inlet for the hot liquid and exits from the opposite corner of the same end as the outlet for the hot liquid. In other embodiments, the inlet for the hot liquid is located at the opposite corner on the same side as the outlet for the cool liquid and the outlet for the hot liquid is located at the opposite corner on the same side as the inlet for the cool liquid.

FIG. 2B illustrates an individual hot panel (2) while FIG. 2C illustrates an individual cold panel (3). Each of the hot panels (2) and cool panels (3) have an impermeable base (12). Suitable materials for the impermeable base include heat stable substances capable of efficient heat transfer such as metals and metal alloys (e.g. aluminum, copper, brass, steel, bronze, etc.). In some embodiments, however, metals such as stainless steel, alloys of iron and chromium, lead, and titanium are preferred. An open celled metal foam material (9), such as DUOCEL® is located on the impermeable base. The open celled metal foam material is heat stable and capable of efficient heat transfer and is typically made of a low temperature alloy including, but not limited to, aluminum, carbon, copper, platinum, silicon carbide, and zinc. The open celled metal foam material is placed on the impermeable base such that fluid enters the panel through an inlet (5, 7), flows into and must pass through the open celled metal foam material (9) prior to exiting the panel at an outlet (6, 8). In some embodiments, the open celled metal foam material (9) is centered on the impermeable base such that an open space (12, 13) exists between the impermeable base (11) of the panel, the impermeable container (10), the open celled metal foam material (9), and either the impermeable base (11) of the panel above or the top of the impermeable container (10) encasing the open celled foam metal counter flow heat exchanger (1) (see FIG. 1B). In other embodiments, the fluid inlet (5, 7) enters the open celled metal foam material (9) directly.

FIG. 3 illustrates the structure of a cell (15) of a relative density continuous one-piece insoluble reticulated open cell foam material (9) prior to densification. Typically, each cell (10) is a three-dimensional 14-faceted polyhedral (tetrakaidekahedron) structure. Each cell (10) is defined by ligaments (16) which create a pore (17); however, because the ligaments (16) are interconnected, each pore (17) is a component of at least two cells (15). The resulting structure is there for identical in all three directions and is considered isotropic. Consequently, because all of the pores (17) are interconnected, fluids are able to pass freely into and out of the open celled foam material (9).

FIG. 4 illustrates a single cell (15) of a relative density continuous one-piece insoluble reticulated open cell foam material (9) after densification. The relative density controls the cross-sectional shape of the ligaments (16), as shown in FIG. 5. As can be seen in FIG. 5, while the number of pores of an open celled metal foam material (9) remains constant, the cross-sectional shape of the ligaments (16) varies depending upon the relative density. Moving from a low density (e.g. 3-5%) to a higher density (e.g. 11-13%), the ligaments transition from a triangular prism shape with sharp corners through an intermediate triangular prism with rounded corners and culminating in almost a perfect cylindrical shape.

Currently, managing pressure drop during thermal design of heat exchangers is a significant problem. Ideally, the calculated pressure drop is within and as close as possible to the allowable pressure drop. In the invention, the fluid flow passes through a field of open celled foam metal material, such as DUOCEL®, which provides enhanced material coverage while reducing pressure drop. The cross field flow of hot and cool fluids allows precise selection of pore numbers and ligament geometry for enhanced performance, especially in situations where turbulent and laminar flow fields differ given viscosity and Reynolds numbers. For high pressure systems, improved results are obtained with open celled metal foam having 40 pores per inch (PPI) and 7-8% relative density and is compressible

The open celled foam metal counter flow heat exchanger (1) can be manufactured at low cost using standard vacuum brazing, dip brazing and/or casting techniques known in the art. The open celled foam metal counter flow heat exchanger (1) of the invention is suitable for use in jet engines, car engines, and electronic cooling structures. 

1. An open celled foam metal counter flow heat exchanger comprising a. an impermeable housing container; and b. a combination of at least two adjacent panels, each panel comprising i. an impermeable base; ii. a field of open celled foam metal comprising cells comprised of ligaments and pores; iii. a fluid inlet; iv. a fluid outlet; and v. optionally, at least one fluid channel.
 2. The open celled foam metal counter flow heat exchanger according to claim 1, wherein each panel has the fluid inlet located on the same end as the adjacent panel.
 3. The open celled foam metal counter flow heat exchanger according to claim 1, wherein each panel has the fluid outlet located on the same end as the adjacent panel.
 4. The open celled foam metal counter flow heat exchanger according to claim 1, wherein each panel has the fluid inlet located on the opposite end as the adjacent panel.
 5. The open celled foam metal counter flow heat exchanger according to claim 1, wherein each panel has the fluid outlet located on the opposite end as the adjacent panel.
 6. The open celled foam metal counter flow heat exchanger according to claim 1, wherein the field of open celled foam metal has ligament geometry to enhance turbulent and laminar fluid flow.
 7. The open celled foam metal counter flow heat exchanger according to claim 1, wherein the open celled foam metal has 40 pores per inch (PPI) and 7-8% relative density and is compressible.
 8. The open celled foam metal counter flow heat exchanger according to claim 1, wherein the open celled foam metal is DUOCEL®.
 9. A method of making the open celled foam metal counter flow heat exchanger according to claim 1 comprising combining fluid flow fields to create improved heat transfer performance.
 10. A method of using the open celled foam metal counter flow heat exchanger according to claim 1 comprising a. inserting a first liquid into a first panel of the combination; b. moving the first liquid through the field of open celled foam metal and removing the first liquid from the first panel; c. inserting a second liquid into a second panel of the combination; d. moving the second liquid through the field of open celled foam metal; and e. removing the second liquid from the second panel, wherein the flow of the second liquid is in the opposite direction as the flow of the first liquid and wherein heat has been exchanged between the first liquid and the second liquid.
 11. The method of using the open celled foam metal counter flow heat exchanger according to claim 10, wherein the combination comprises the first panel adjacent to the second panel.
 12. The method of using the open celled foam metal counter flow heat exchanger according to claim 10, wherein the combination comprises a series of first and second panels.
 13. The method of using the open celled foam metal counter flow heat exchanger according to claim 10, wherein the first liquid is a cool liquid and the second liquid is a hot liquid.
 14. The method of using the open celled foam metal counter flow heat exchanger according to claim 10, wherein the first liquid is fuel and the second liquid is oil. 